The invention relates broadly to the field of flowmeters and particularly to acoustical flowmeters for measuring the rate of flow of a fluid along a confined path.
A typical acoustical wave flowmeter is disclosed in U.S. Pat. No. 3,109,112. This flowmeter has a pair of transducers for generating and receiving compressional waves in either the audible or ultrasonic frequency range. These transducers are located within an enclosure through which the fluid flows. The transducers are alternately configured in transmit and receive mode so that compressional waves are produced in the gas by the transmitting transducer and received at the other transducer. By measuring the phase difference between the transmitted and the received waves in both directions, the velocity of the gas passing through the transducer is determined.
A modification to the approach described in the above mentioned U.S. Patent relies on transducers designed with a transmit and a receive section configured to substantially eliminate any problems due to gas composition or temperature changes in the gas.
The above known configuration requires that the transducer be located in the flow path of the gas whose velocity is to be measured or located in a cavity in the conduit wall. In either arrangement, the normal flow of the fluid is substantially altered when passing the transducer thereby causing the accuracy of the meter to suffer. Additionally, sedimentery particles in the fluid or the like can collect around the transducers thereby impairing the transducer's transmitting and receiving characteristics.
A flowmeter has been developed in an effort to overcome these and other difficulties of known acoustical flowmeters. The improved flowmeter is designed to have no obstructions along the flow path and is described in U.S. patent application Ser. No. 599,245 filed on July 24, 1975, entitled "Acoustical Wave Flowmeter" and invented by Edward James DeWath, now U.S. Pat. No. 4,003,252. In that flowmeter, the transducers are generally cylindrical in shape and are disposed within the walls of the fluid conduit thereby eliminating all obstructions in the flow path as well as eliminating cavities in the conduit wall in which debris might collect.
While the invention of Edward James DeWath does eliminate obstructions in the path of fluid flow as well as cavities in the conduit wall, his flowmeter may become inaccurate or fail to function entirely whenever the composition of the gas flowing therethrough is different from that for which the meter is calibrated. Indeed, this is a common problem with most acoustical flowmeters as the velocity of acoustical compressions within a gas is a function of its chemical composition so that typical acoustical flowmeter accuracy depends on calibrating it for the specific gas whose flow is to be measured. Requiring recalibration each time the flowmeter is used with a different gas is, at best, an inconvenience. In applications where the gas composition changes as the flowmeter is used as, for example, in pulmonary function analyzers, known acoustic wave flowmeters are inaccurate unless corrective feedback is provided from a gas analyzer to compensate for gas composition change.
Accordingly, it is an object of the present invention to provide a flowmeter which is accurate regardless of changes in gas composition or temperature during use.
It is yet another object of the invention to provide a flowmeter including means to automatically adjust the flowmeter operation as a function of dynamic gas compositional and temperature changes and to provide a measure of the velocity of sound, which is itself related to the average molecular weight of the gas.